1. Field of the Invention
The present invention relates to a semiconductor memory which requires a refresh operation periodically to retain data written in a dynamic memory cell. More particularly, the invention relates to a semiconductor memory which performs the refresh operation internally and automatically without requiring refresh commands from the exterior.
2. Description of the Related Art
In a pseudo SRAM having a dynamic memory cell and a DRAM having a self-refreshing mode, it is necessary to refresh the memory cell periodically with an internal circuit to retain data written in the memory cell. Specifically, this type of semiconductor memory incorporates a refresh timer which generates a refresh request signal for performing the refresh operation. Generally, a refresh timer includes an oscillator which generates an oscillation signal, and a frequency divider which periodically generates a refresh request signal by dividing the frequency of the oscillation signal.
The frequency of the oscillation signal is changed along with the fluctuation of fabricating conditions of the semiconductor (such as the fluctuation of the properties of the transistor and the like) and an operation environment of the semiconductor. For example, when the frequency of the oscillation signal becomes low and the refresh cycle is prolonged, there arises a possibility that data in a memory cell is lost before the refresh operation is performed. In order to prevent such a drawback, a frequency divider having a variable dividing ratio has been proposed (for example, Japanese Unexamined Patent Application Publication No. 2003-30983, Japanese Unexamined Patent Application Publication No. Sho 63-241790). By decreasing the dividing ratio when the frequency of the oscillation signal is low, it is possible to prevent the generation cycle of the refresh request signal from being prolonged. As a result, the drawback that the data in the memory cell is lost is prevented.
In recent years, a DRAM or a pseudo SRAM has been used as a work memory of a portable equipment. The portable equipment is required to exhibit the low power consumption to prolong a use time of a battery. Particularly, it is important that a standby current while the portable equipment is not in operation is low.
For lowering the power consumption of a refresh timer and the standby current, the frequency of the oscillator may be lowered. For example, by lowering the frequency of an oscillation signal to a value equal to the frequency of a refresh request signal, the power consumption of the refresh timer is largely reduced. However, to vary the frequency of the refresh request signal without using a frequency divider, it is necessary to change the oscillation frequency of the oscillator. To vary the frequency of the oscillation signal, a circuit scale of the oscillator is enlarged. Particularly, since the oscillator signal is used as a standard signal, the frequency of the oscillation signal is required to have a certain accuracy.
Therefore, this makes the circuit design of the oscillator complexed as well as the testing of the oscillator. Accordingly, it is undesirable to vary the oscillation frequency of the oscillator.